Endometriosis is another one of those female "complaints" just beginning to get a lot of attention from medical researchers.

A few years ago, endometriosis was dubbed "the career woman's disease," because, the doctors believed, the apparent increase in the number of sufferers was related to the phenomenon of more women waiting to have their first children until after their careers were established.

Now, however, some specialists believe that much of the "increase" may be accounted for by better diagnoses, especially in women who experience difficulty in conceiving after having waited to have a family. Perhaps one in 20 women has endometriosis, according to experts at the National Institutes of Health. Other specialists suggest it may be as many as one in 10. It seems to be implicated in about 20 percent of all infertility problems.

The experts still do not know the cause of endometriosis, but there is now some progress in its control, especially in reversing its adverse effect on the fertility of young women.

The endometrium is the lining of the uterus. Each month, responding to instructions from brain and pituitary gland hormones, and, in turn, those from the ovaries, the endometrium is readied physiologically to receive and nurture a fertilized egg: It thickens to about 10 times its initial size. If no fertilized egg is produced, the thick lining breaks apart, sloughs away and is discharged by means of the menstrual period.

Sometimes endometrial tissue attaches itself to and grows in other parts of the reproductive system, in the abdomen or the intestines--almost anywhere. The same hormones that cause changes within the uterus also cause changes in endometrial tissue--wherever it is--but while the uterine tissue is expelled during menstruation, the other tissue has no place to go. Pain and scarring may occur when the body tries to reabsorb these wandering patches of uterine tissue, or surround them with extra prostaglandin-induced inflammation.

Endometriosis, however, may occur with no painful symptoms. But commonly it produces severe dysmenorrhea (menstrual cramps) and also may cause pain or discomfort during intercourse.

The breakthroughs in technology and biochemical knowledge that have permitted the implantation in a woman's uterus of an egg fertilized in a glass dish also have provided more successful means for diagnosing and treating endometriosis. Both microsurgery and hormonal manipulation can be used. The presence of endometriosis can be confirmed positively only by a surgical procedure called a laparoscopy, which is usually minor surgery, but requires a general anesthetic.

In some patients the use of birth- control pills seems to keep the disorder under control. The pills bypass the body's normal system of hormonal feedbacks, keeping the endometrium considerably thinner than it normally would be.

Because the disorder becomes a specific problem when a woman begins to think about having a family, much of the research has been keyed to overcoming its effect on fertility.

One drug that has had some success is danazol, but even here the success rate is limited--only 40 percent of those treated become pregnant--and there can be unpleasant side effects.

Danazol--the brand name is Danocrine--works by decreasing the production of the pituitary gland hormones that stimulate the ovary. The ovaries, in turn, lower their production of estrogen, the hormone that ultimately stimulates the endometrium.

Some women treated with danazol gain weight--an average of about nine pounds, mostly through fluid retention. In addition, the drug can cause acne and, because it contains some masculine hormonal activity, may cause the growth of hair on lips, chin and sideburn areas. It also may reduce breast size, cause vaginal dryness and even transient hot flashes. These symptoms disappear when use of the drug is discontinued.

Although danazol is currently considered the best drug treatment for the condition, the National Institute of Child Health and Development is about to launch a study of a drug that may be even more effective with fewer side effects.

The drug, called LHRH analog, has a chemical structure similar to the brain hormone, Luteinizing Hormone Releasing Hormone (LHRH). In effect, when taken continuously, it befuddles the feedback systems of a female's reproductive cycle enough to keep the endometrium from being stimulated.

Close relatives of this drug have already been used with some success--and no apparent harmful side effects--to treat children who develop sexually early, under age 10. It has been used to treat some prostate cancers in men and as a possible contraceptive for women.

Study participants--only those with confirmed endometriosis--will be given either LHRH, danazol or a placebo for a period of six months. The LHRH will be given initially through daily self-administered injections and replaced eventually by a nasal inhalant. (All participants will use injections/inhalants and pills so that neither patient nor health-care practitioner will know who is getting what drug or which participants are receiving only the placebo.)

The NIH study, in which all services and medications are free, will involve 150 women age 20 to 35 with infertility caused by mild to moderate endometriosis, as diagnosed on the basis of a laparoscopy procedure. (Some of these procedures may be done at NIH.)

Those selected will undergo a six-month course of treatment and will be followed for one to five years thereafter, depending on their pregnancy rates after treatment. (Women who receive only the placebo initially will be offered the new drug, or danazol, free of charge for as long as they need it after completion of the double-blind portion of the study.

Women interested in participating in the study should have their physicians write a letter of referral to: Dr. Abraham Munabi, National Institute of Child Health and Human Development, Building 10, Rm. 10B09, National Institutes of Health, Bethesda, Md. 20205.

For free booklets and more information on endometriosis, write: American College of Obstetricians and Gynecologists, 600 Maryland Ave. SW, Washington D.C. 20024.